The invention relates to a process for preparing carbamatoorganosilanes from corresponding chloroorganosilanes, metal cyanates and alcohols.
The prior art discloses various processes for preparing 3-carbamatopropylsilanes of the formula (1).

Common preparation processes usually proceed from 3-aminopropylsilanes of the formula (2). These can be reacted either with dialkyl carbonates or else a mixture of urea and alcohol to give the corresponding carbamatosilanes. The former process variant is described, for example, in WO 2007/037817 and the latter in EP 1010704.

A disadvantage of this process is the fact that it is necessary to proceed from aminosilanes, which are usually much more expensive than the corresponding 3-chloropropylsilanes of the formula (3).

In addition, only aminoorganosilanes with a propyl spacer between the amino and silyl groups are commercially available, while the corresponding α-aminomethylsilanes of the formula (5) are obtainable only with difficulty and are additionally only of moderate stability. Accordingly, the abovementioned processes are unsuitable for preparation of the α-carbamatomethylsilanes of the formula (4), which are of interest for many (conversion) products. The corresponding α-chloromethylsilanes of the formula (6), in contrast, are preparable without any problem, for example via the photochlorination, described in EP 1310501, of methylchlorosilanes and subsequent alkoxylation, i.e. the exchange of all silicon-bonded chlorine atoms for alkoxy groups.

Thus, a process with which a wide range of different carbamatoorganosilanes is obtainable proceeding from the corresponding chloroorganosilanes would be desirable.
Corresponding processes have likewise already been described, for example in U.S. Pat. No. 3,494,951. Here, a mixture of 3-chloropropylsilanes of the formula (3) is heated with potassium cyanate and an alcohol in a solvent under reflux. The solvent used is preferably dimethylformamide. This forms 3-carbamatopropylsilanes of the formula (1), and potassium chloride as a coproduct. The latter is filtered off and the solvent is removed by distillation.
However, it has been found that this process in accordance with the prior art has numerous distinct shortcomings. Firstly, on account of the comparatively long reaction times and the necessity to use large amounts of solvent, only very poor space-time yields are achieved. In addition, it was found that the removal by filtration of the salt formed as a coproduct is exceptionally problematic since the salt is obtained in such finely crystalline form that the reaction mixture can be filtered only with great difficulty. Finally, it has also been found that the filtered crude product solution also still contains significant amounts of dissolved salt. These then precipitate out under cold conditions (for example when the crude product is stored in unheated rooms in winter) and/or when the solvent is removed, and disrupt the further process steps, for example distillative purification of crude product by means of a thin-film evaporator.
A further process for preparing carbamatoorganosilanes of the formula (1) or (4) from the corresponding chloroorganosilanes (3) or (6) is described in DE 10240388. Here, a suspension of the metal cyanate in a solvent is initially charged and a mixture of methanol and chloroorganosilane is added dropwise. The solvent used is preferably dimethyl sulfoxide or dimethylformamide. Here too, the respective carbamatopropylsilanes form, with potassium chloride as a coproduct. The latter is again removed by filtration, and the solvent is removed by distillation.
However, in this process variant too, various disadvantages were identified. Firstly, relatively large amounts of polymeric and oligomeric by-products are formed. This by-product formation is contradictory to the details in DE 10240388, which describes quantitative yields with product purities of approx. 95%. However, these relatively high molecular weight impurities are not detectable by the gas chromatography analyses, described in DE 10240388, of the reaction mixture, and therefore gas chromatography spectra give correspondingly high purities even when the product solution contains 10-20% oligomers or polymers.
Secondly, in the process described in DE 10240388, the problems of inadequate filterability of the reaction mixture and of the salt burden remaining dissolved in the filtrate are also unsolved.
It was therefore an object of the present invention to develop a process for preparing carbamatoorganosilanes proceeding from chloroorganosilanes which no longer has the disadvantages of the prior art.